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Dead water: the worst nightmare of boats and sailors

marzo 28, 2022

A study conducted in France finally explains the phenomenon that has stumped oceanographers since 1893.

A reality that many know but few wonder how it works. Even after more than a century it can go consciously unnoticed. This is the mysterious effect that leaves ships stopped in the middle of the sea even if they have their engines running.

In 1893, the explorer Fridtjof Nansen began an expedition to the North Pole that would give him world fame for breaking a navigation record, but in the middle of his journey he was able to observe a strange phenomenon that has puzzled oceanographers for more than a century: When was sailing through the waters of the Arctic, north of Siberia, his ship began to stall even though its engines were at full throttle.

“We did loops in our course, sometimes we went around, we tried all kinds of strategies to avoid it, but with very little success,” Nansen recounted.

Nansen thus became the first to notice the phenomenon he named “dead water”.

Fridtjof Nansen

Eleven years later, in 1904, the Swedish physicist and oceanographer Vagn Walfrid Ekman managed to identify what was causing this anomaly.

Ekman showed in a laboratory that waves formed in this part of the Arctic Ocean below the surface, between layers of salt and fresh water (which have different densities) interacted with a ship, generating drag.

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The water in the Arctic is mixed with various layers of salinity.

The origin attributed to the melting glaciers, forming a layer of fresh water over the salty sea water. The saltier the denser. However, in his laboratory tests, Ekman saw that the drag waves generated oscillations in the ship’s speed.

This differed from the observations of Nansen, whose ship stopped at a constant and abnormally low speed. No one had been able to explain the differences, or understand exactly how the dead water effect worked.

But an interdisciplinary team from the National Center for Scientific Research (CNRS), the most important research institution in France, and the University of Poitiers, believes they have unraveled both mysteries.

In a study published in July in the scientific journal PNAS, they concluded that the variations in speed are due to the generation of waves that act as a kind of “undulating conveyor belt”.

The dead water traps the boats and causes them to bob as if they were on the tape, which causes them to move back and forth. Unifying the observations of Ekman and Nansen, the scientists say that the effect is only temporary.

“Finally, the boat ends up escaping and reaches the constant speed that Nansen described,” they published in their study.

The experts highlighted that the phenomenon does not only occur in places with glaciers, but in all the seas and oceans where waters of different densities mix.

“It is also found in cold mountain lakes in summer because there is temperature stratification, and therefore there is a risk of drowning for swimmers,” said study co-author Germain Rousseaux.

Rousseaux added that the phenomenon also occurs at the mouth of rivers such as the Orinoco, in South America, due to the flow of the rivers with sediments on the salty sea water.

Curiously, this study was not carried out in order to unravel the mystery of what happened to Nansen more than a century ago, but to unravel a much older mystery.

Was Cleopatra and Antony’s fleet trapped in dead water during the Battle of Actium?

The work is part of a large project that investigates why during the Battle of Actium or Actium (in the year 31 BC), in ancient Greece, the great ships of Cleopatra and Marco Antonio lost when they faced the ships Caesar Octavio’s weakest.

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